hdf5_ep

[svn-r10797] Purpose: A feature

Description:  Cygwin compiler doesn't do rounding correctly when converting
"unsigned long long" to "long double".

Solution: Added test case to variable detection of
"hdf5_cv_ullong_to_ldouble_precision_works".

Platforms tested: sleipnir, fuss, and shanti - mainly to test configuration,
don't need to run h5committest.